you can also forget the walls,

 

and use my drilling machine vertically, fed power by a long line.

 

then simply send in another machine that will scoop up the loosened minerals in contact woth the drilling machine.

 

then, instead a straight line dig,

 

it should be like a tractor loosening up the soil.

 

then if you remember my sea floor leveling tool,

 

my previous drilling machine, can be modified to,  have each layer as rings of a long cylinder,

rotating once it is drllied to that cylindrical height.

 

and then from there, it would need to move side ways from the dirt so,

 

the very top in surface contact, can be creating a leverage placement, to anchor itself pushing away from the fixated point.

 

OR,

use my multi-bucket digging tool (following the world's largest digging machine design)

and loosen up the soil creating a large field-like circle,

just like a diamond mine? or whichever mine that you can descend down to incrementally...

 

no wait it don't need wheels nor does it need to create a circle-field.

it is simply dropped by a powerline.